Method and apparatus for preventing breakage of liners



.Iune 26, 1 9 62 .1. E. HICKS ETAL 3,041,133

METHOD AND APPARATUS FOR PREVENTING BREAKAGE OF LINERS Filed Sept. 27,1960 4 I8 I0 7 F lllll l l lll INVENTORS 7 John E. Sigler 8 John E.Hicks ATTORNEYS This invention relates to the production andpurification of silicon crystals and more particularly to a method andapparatus for producing silicon crystals in a graphite crucible having aquartz liner in a manner to prevent the breakage of the quartz liner.

Crystals of various chemical elements and compounds such as germanium,silicon, and the like are used in semiconductor devices which areassuming a role of increasing importance in the electronic industry.Various methods are in use today for producing and refiningsemiconductive materials having the essential properties of high purityand homogeneity. However, in the refining of such materials,difficulties may be present when working with one particular metal ormaterial which may not occur when working with another. For instance,when working with germanium, which has a relatively low meltingtemperature of approximately 980 C., the material can be handled easilyin graphite containers or crucibles, and does not appear to attack thegraphite container appreciably during the refining processes. Also,germanium can be melted and solidified in a graphite crucible withoutdestroying the crucible upon subsequent cooling. In addition, graphiteis a very common material and is easily machined. On the other hand,silicon has a much higher melting point, about 1420 to 1440 C., andgraphite is unsuitable as a container to hold the material during therefining processes. Experimentation has shown that although quartz isthe most acceptablecrucible material at the present time, at these hightemperatures an undesired reaction oocurs between the quartz and thesilicon. If a portion of the molten silicon is allowed to solidify inthe quartz crucible, bonding of the silicon to the quartz occurs and thecrucible will shatter or crack upon cooling due to the difference inthermal expansion and contraction of the two materials.

During the melting of a silicon mass within a quartz crucible orcontainer, there usually results a small amount of leaching ofimpurities from the quartz container, such as boron, and it is readilyapparent that if the quartz container or crucible could be recovered andreused after the initial operation, any subsequent operation with thesame quartz container would occur with a substantial reduction in theamount of leaching of such impurities from the container. It is believedthat after a number of repeated reuses of the same quartz crucible,leaching of the impurities, such as boron, from the container by thesilicon mass during the refining operation would be reduced. 1

In a copending application of Walter R. Runyan, Serial No. 758,379,filed September 2, 1958, and assigned to the assignee of the presentinvention, a method and apparatus for producing and refining siliconmaterials is fully described and claimed. 'In accordance with one formof the invention of the copending application, there is provided agraphite crucible or container having a quartz liner in which thesilicon is melted prior to the formation of the end product. A smallaperture is formed in the bottom of the quartz liner which is smallenough to prevent the escape of the molten silicon under normalgravitati-onal force; After the silicon material is completely meltedand homogeneous, and the Well-known crystal pulling process is employedfor producing a single e atnt crystal of high purity silicon, the moltenmass remaining in the quartz liner is removed by increasing the pressureabove the upper surface of the material so as to force the materialthrough the aperture. The crucible is then free of any residue silicon,and may be reused in subsequent operations. However, when the crucibleis permitted to cool, it has been found that the diflerence in theexpansion coefficients of quartz and graphite during cooling cause thevaluable quartz liner to become broken.

It is an object of the present invention to prevent the breakage of sucha crucible liner by providing a method and apparatus for lifting theliner out of contact with the crucible during cooling.

it is another object of the present invention to provide a method andapparatus utilizing a cushion of air for lifting a quartz liner out ofthe supporting well of a graphite crucible to prevent breakage of thequartz liner during cooling.

It is a further object of the invention to provide a method andapparatus for preventing the breakage of quartz liners during coolingwhich is simple and effective in operation, economical, and does not.require mechanical engagement or contact with the liner.

Further objects and advantages of the present invention will becomereadily apparent as the following detailed description of the inventionunfolds and when taken in conjunction with the drawings wherein:

FIGURE 1 is a fragmentary sectional view of a crucible embodyingfeatures of the present invention; and

FIGURE 2 is a fragmentary sectional view similar to that of FIGURE 1illustrating the quartz liner in its raised position out of engagementwith the supporting wall of the crucible.

Referring to FIGURE 1, one embodiment of the present invention comprisesa crucible 4 made of a suitable material, such as graphite, which issupported on the upper end of the tubular support 6 preferably made ofthe same material as the crucible 4. A quartz liner 8 is supportedwithin a cavity 10 in the upper surface of the crucible 4. Suitablethermocouple tubes 12 may also be provided for sensing the temperatureof the crucible.

An aperture 14 with a downwardly flared annular lip 16 is provided inthe bottom of the quartz liner 8, and a larger opening 17 is formed inthe bottom of the crucible 4 to enable the aperture 14 to communicatedirectly with the central opening in the tubular support 6. As

described in the aforementioned copending application,

the silicon material is melted in the crucible. The diameter of theaperture 14 is small enough to hold the molten silicon in the liner bysurface tension, thereby preventing leakage. A diameter in the order ofA to of an inch has been found to produce satisfactory results with theconventional crystal pulling apparatus.

After an ingot of silicon is melted in the liner 8 and a single crystalof the material is formed by the conventional pulling operation, asdescribed in the copending application, a small residue 18 of siliconremains. To remove the residue from the liner, the pressure of an inertgas above the residue 18 is increased to a value sufficient to overcomethe surface tension of the residue with respect to the aperture 14. Agas pressure of 2" to 3" of mercury (Hg) was found to be sufiicient toproduce the desired results. Suitable cooling and collecting means (notshown) are provided beneath the tubular support 6 to quickly solidifyand collect the silicon after it passes through the aperture 14.

After the residue 18 is completely cleared from the crucible, and thecrucible is permitted to cool, the difference in the expansioncoeflicients of the quarts liner 8 and the graphite crucible 4 can causethe liner to become broken. The present invention prevents the breakageof the quartz liner 8 during this cooling period by lifting the linerupwardly to the position illustrated in FIGURE 2 by increasing the gaspressure beneath the liner 8. Thus, the liner is in eifect floatedupwardly on a cushion of gas. A pressure of 2 to 5 psi. is usuallysufiicient t0 float the liner 8. When introduced below the liner, someof the gas will escape upwardly through the aperture 14, but enough ofthe gas will be directed around the annular lip 16 and between the liner8 and the wall of the cavity 10 to float the liner upwardly to theposition illustrated in FIGURE 2.

The gas pressure may be increased beneath the liner 8 in any suitablemanner but, in accordance with one em bodiment of the invention, gas isintroduced into the tubular support 6 through a threaded plug 20threaded into the wall of the tubular support. A suitable conduit 22 isdetachably connected to the threaded plug 20 by a nut 24 on the end ofthe conduit which threadably engages the left end of the threaded plug.A suitable source 26 of pressurized gas may then be connected to theconduit 22 by a conduit 28 with a valve 31 disposed therebetween toregulate the flow and pressure of the gas. With this construction, gascan be introduced into the central opening of the tubular supportbeneath the liner 8 to float the liner upwardly as previously described.When the crucible and liner have cooled sufficiently, the gas pressureis reduced to allow the liner to rest in the crucible 4.

Thus, the present invention provides a method and apparatus foreffectively and dependably preventing breakage of the quartz liner. Theliner is very gently floated upwardly and lowered on a cushion of gaswhich eliminates damage to the liner that might occur if it werephysically lifted by some type of mechanical gripping means. Further, byintroducing the gas from beneath the liner, the need for apparatus whichwould require valuable head room above the cavity to perform the liftingoperation is eliminated. This avoids interference with crystal pullingapparatus which may be positioned above the cavity 10.

If desired, the apparatus of the present invention can also be utilizedto decrease the gas pressure beneath the liner 8 by connecting a conduit32 to a suitable evacuating device, for example. In this manner, thepressure differential across the aperture 14 required to force theresidue 18 out of the liner can be created, and this will eliminate theneed for providing the means necessary to increase the gas pressureabove the cavity 10 to remove the residue 18, as previously described.

Although the present invention is especially useful in the production ofsilicon, it would find utility in any application in which it isdesirable to physically separate the crucible and crucible liner. Theabove-described method and apparatus are specific embodiments of theinvention, and many modifications may be made thereto without departingfrom the spirit and scope of the invention, which is limited only asdefined in the appended claims.

What is claimed is:

l. The method of preventing the breakage of a quartz liner of a crucibleduring cooling, which includes the step of introducing pressurized gasbetween the liner and the crucible to float the liner in spaced relationfrom the crucible during the period that the liner and crucible arecooling.

2. The method of preventing the breakage of the quartz liner of acrucible during cooling, said crucible having an aperture at the bottomthereof, said liner having an annular lip at the bottom thereofenclosing an aperture extending intothe aperture of said crucible, saidlip being positioned in spacedrelation with the wall of said crucible,which includes the step of introducing pressurized gas between saidcrucible and said liner through the'space between said crucible wall andsaid annular lip, whereby the gas, working its way between the liner andthe crucible, gently lifts the liner upwardly out of engagement with thecrucible.

3. The method of preventing the breakage of a quartz liner duringcooling, said crucible having an aperture at the bottom thereof, saidliner having an annular lip at the bottom thereof enclosing an apertureextending into the aperture of said crucible, said lip being positionedin spaced relation with the wall of said crucible, which includes thesteps of introducing pressurized gas between said crucible and saidliner through the space between said crucible wall and said annular lip,whereby the gas, working its way between the liner and the crucible,gently lifts the liner upwardly out of engagement with the crucible, andremoving said pressurized gas after the liner and crucible have cooledsufficiently, thereby to gently lower the liner back into engagementwith the crucible.

References Cited in the file of this patent UNITED STATES PATENTS

3. THE METHOD OF PREVENTING THE BREAKAGE OF A QUARTZ LINER DURINGCOOLING, SAID CRUCIBLE HAVING AN APERTURE AT THE BOTTOM THEREOF, SAIDLINER HAVING AN ANNULAR LIP AT THE BOTTOM THEREOF ENCLOSING AN APERTUREEXTENDING INTO THE APERTURE OF SAID CRUCIBLE, SAID LIP BEING POSITIONEDIN SPACED RELATION WITH THE WALL OF SAID CRUCIBLE, WHICH INCLUDES THESTEPS OF INTRODUCTING PRESSURIZED GAS BETWEEN SAID CRUCIBLE AND SAIDLINER THROUGH THE SPACE BETWEEN SAID CRUCIBLE WALL AND SAID ANNULAR LIP,WHEREBY THE GAS, WORKING ITS WAY BETWEEN THE LINER AND THE CRUCIBLE,GENTLY